This invention relates in general to apparatus and a process for the flotation clarification of water. More specifically, it relates to an apparatus and process where multiple such processes are carried out simultaneously in a single tank to provide an extremely high treatment rate with a high degree of compactness.
Water clarification, the removal of particulate contaminants suspended in water, is used to treat waste water from manufacturing processes, particularly in the pulp and paper industries, in the treatment of municipal water drinking supplies, and in sewage treatment. The water can be clarified by sedimentation or flotation of the particles. Known sedimentation techniques and apparatus are efficient, but even at peak efficiency they are limited to a comparatively slow sedimentation rate, about 0.5 gallons per minute per square foot. To handle large volumes of raw input water, sedimentation facilities must therefore be large, with attendant cost and space utilization disadvantages.
Flotation techniques dissolve a few percent of air by volume in pressurized water and then release the air in the form of microscopic bubbles which attach to the particles and carry them to the surface where they form a floating sludge. The particles are usually flocculated using conventional flocculating agents such as alum. Flotation techniques are theoretically capable of achieving clarification rates of 7.5 gallons per minute per square foot of flotation area.
Applicant holds several U.S. patent on apparatus and processes for water clarification by flotation. U.S. Pat. Nos. 2,874,842 and 3,182,799 introduce gas bubbles to a tank with no moving components inside or over the tank. Internal deflectors and baffles guided the water and the flotation process; the turbulence of the water was minimal and did not present a design problem. This equipment, however, operated slowly.
U.S. Pat. No. 4,022,696 describes an improved clarifier of applicant sold commercially under the trade designations "SPC", "Supracell", and "Supercell". Flotation occurs in a circular tank. Raw water is fed from a raw water inlet pipe at the center of the tank, a hydraulic joint, and a manifold that rotates about the tank. The inlet flow from the manifold is directed opposite to the direction of rotating so that the raw water entering the tank has a net zero velocity. Flocculation occurs in a flocculation chamber, an assembly of pipes, walls and baffles that is immersed in the tank and rotates on a carriage with the manifold. Another rotating manifold introduces dissolved air to the flotation tank following the flocculation assembly. A rotating scoop, preferably a multiple blade scoop of the type described in applicant's U.S. Pat. No. 4,184,567 is also carried on the carriage. It scoops the floated sludge from the surface of the tank and directs it to a central pipe that is inclined to gravity feed the collected sludge to a central collection compartment.
The carriage rotates about the tank at a rate such that the floated particles reach the surface in about one rotation. With a flotation rate of 12 inches per minute, a good degree of clarification can be achieved with each rotation using a comparatively shallow tank, e.g. 16 to 18 inches of water. While these units, operated in this manner, are much more efficient than the earlier stationary equipment, and in general they are highly successful, they do not provide a throughput capacity that can handle a large municipal treatment plant without utilizing a large number of the apparatus. For example, to treat raw water input at 8 to 9 m.sup.3 /sec would require more than twelve SPC units of the largest size, a 70 foot (about 22 m) diameter which processes 11,800 gallons per minute, or 0.73 m.sup.3 /sec. This many units has serious disadvantages in terms of capital cost, operating cost, and space utilization. Further, simply increasing the size of each unit and operating it at a faster speed does not provide an acceptable solution since the increased speed of rotation of the carriage required to take advantage of the increased size creates turbulence as the flocculation assembly and dissolved air manifold move through the water with an increased velocity. Also, a net zero velocity operation is more difficult to achieve at higher rotational speeds.
U.S. Pat. No. 4,377,485 discloses a clarifier (sold under the trade designation "SAF") operating with the same features as the SCP unit, but adding a second filtration stage, a set of sand beds covering the bottom of the tank. While the clarified water is cleaner than with a comparable SPC unit, the SAF has a higher initial cost and does not increase the filtration rate.
U.S. Pat. No. 4,626,345 discloses a further clarifier sold under the trade designation "SASF". This unit does not employ the SPC's zero velocity principle, but it does offer two stage filtration with sand at a favorable cost and compactness as compared to the SAF unit. Raw water in this unit enters a central cylindrical compartment at the bottom. This compartment acts as a hydraulic flocculator. Aerated water is introduced to this central compartment by a fixed manifold located near the upper end of the compartment. Flocculated and aerated water is fed to a clarification tank by flowing over the upper edge of the cylindrical compartment into the surrounding tank. This unit provides good clarification with a compact unit, but it does not offer the throughput capacity or overall performance of the SPC or SAF units.
U.S. Pat. No. 4,931,175 discloses another SPC-type clarifier where a set of concentric, conical plates rotate within the tank to increase the length of the path traversed by the rising bubbles. With this arrangement, it is possible to increase the treatment capacity of a given diameter tank, with the increase being roughly approximated by the degree to which adjacent ones of these conical plates overlap one another. A 50% increase in the flotation rate, for example, is well within theoretically possible limits. However, in practice it has been found that it is difficult to obtain consistent increases that equal the theoretical maximum. Moreover, even if the inclined plates worked as well as theoretically possible, they would not increase the treatment capacity of an SPC unit sufficiently to treat raw water at throughput rates as high as 8-9 m.sup.3 /sec, values necessary in certain applications such as the treatment of the municipal waste of a major city by retrofitting existing sedimentation tanks with a fixed area.
It is therefore a principal object of this invention to provide a flotation-type water clarification apparatus and process which has a greatly increased filtration rate for a given diameter tank.
Another principal object is to provide a clarifying system with this greatly increased rate in a compact size that does not require significant additional space, e.g. room for multiple known SPC-type clarifiers.
A further object is to provide a clarifying system with the foregoing advantages that has a comparatively low capital cost and weight as compared to a comparably sized SPC or SAF unit.
Another object is to provide a clarifying system with the foregoing advantages that can retrofit existing sedimentation units.
Yet another object is provide a scoop that removes floated sludge at a high rate and lifts it to promote its movement to a collection compartment.
A still further object is to provide a flotation clarifier with all of the foregoing advantages that has a simplified water distribution and level control that avoids rotary seals, particularly at a raw water inlet feeding flotation tank.